Superconducting stress-engineered micro-fabricated springs

ABSTRACT

A method of manufacturing superconductor structures includes depositing a release film on a substrate, forming a stack of films comprising an elastic material and a superconductor film, releasing a portion of the elastic material by selective removal of the release film so that portion lifts out of the substrate plane to form elastic springs. A method of manufacturing superconductor structures includes depositing a release film on a substrate, forming a stack of films comprising at least an elastic material, releasing a portion of the elastic material so that portion lifts out of a plane of the substrate to form elastic springs, and coating the elastic springs with a superconductor film.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.16/897,667 filed Jun. 10, 2020, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

This disclosure relates to electrical interconnects for computing chips,more particularly to electrical interconnects for quantum computingqubit chips.

BACKGROUND

Many types of quantum computing chips, often referred to as qubit chipsas the term “qubit” means the smallest unit of information for a quantumcomputer, operate at cryogenic temperatures. These requiresuperconducting interconnects. As used here the term “superconductinginterconnect” means an interconnect that comprises elements with zeroelectrical resistance. This typically comprise superconducting materialsoperating at low temperature.

Using non-superconducting interconnects would generate unacceptablelevels of heat. Approaches such as through-silicon-via micro-bumps workwell for connecting conventional chips, but these approaches are notcompatible with forming superconducting interconnects.

Another issue that arises in forming interconnects at cryogenictemperatures lies in the need for mechanical compliance. This allows forshifts in substrate dimensions due to thermal expansion mismatch.Mechanical compliance enables alignment and assembly of disparatepackage components at room temperatures for deployment and operation atlow temperatures. State-of-the-art interconnects typically are notsuperconducting nor are they mechanically compliant to dimension shifts.

SUMMARY

According to aspects illustrated here, there is provided a method ofmanufacturing superconductor structures, including depositing a releasefilm on a substrate, forming a stack of films comprising an elasticmaterial and a superconductor film, and releasing a portion of theelastic material by selective removal of the release film so thatportion lifts out of the substrate plane to form elastic springs.

According to aspects illustrated here, there is provided a method ofmanufacturing superconductor structures including depositing a releasefilm on a substrate, forming a stack of films comprising at least anelastic material, releasing a portion of the elastic material so thatportion lifts out of a plane of the substrate to form elastic springs,and coating the elastic springs with a superconductor film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 shows a series of film depositions to form superconductingsprings.

FIGS. 5 and 6 show alternative embodiments of superconducting springs.

FIG. 7 shows an embodiment of a released thin film structure utilizingbackside metal areas to form contacts.

FIG. 8 shows an embodiment of a stack of films that curl out of theplane of the substrate past 90 degrees.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments here show stress-engineered, superconductormicro-springs for interconnecting cryogenic circuits such as quantumcomputing chips. Generally, these springs form from films deposited on asubstrate, where at least one film has more compressive stress nearerthe substrate than the surface of the film away from the substrate. Thefilm also has higher tensile stress on the surface of the film thannearer the substrate.

The term “spring” as used here refers to structures that curl out of theplane of the substrate. These structures are naturally mechanicallycompliant and can accommodate changes in package dimensions due tothermal expansion mismatch. Curling out of the plane may involve the tipof the spring curling away from the substrate, or the body of the springcurling away from the substrate as will be discussed in more detailfurther.

FIG. 1 shows a substrate 10 having a release film 12 formed on it. Thesubstrate may comprise one of many different types of materialsincluding glass, silicon wafers, flexible substrates or laminates, suchas FR 4, a glass reinforced epoxy laminate. The release film 12 maycomprise any selectively removable material. It lies underneath thestress-engineered film so that when the release layer undergoes removalthe film will curl out of the plane of the substrate. Metallic releasefilms may have an advantage in that they can conduct electricity. Thismay allow for electroplating of any structure connected through therelease layer, and possibly for electrical testing.

The process then forms a stack of films of a stress-engineered film anda superconducting film. FIG. 2 shows the deposition of a first film 14.The first film is deposited directly on the release layer. The firstfilm may comprise a superconductor film, or a stress-engineered film.Both films conduct electricity, so the position of one film relative tothe other is not important. While making contact throughnon-superconductor metals will generate heat, the superconducting metalswill not generate heat, as they have no resistance at cryogenictemperatures. One should note that the stack of films may only compriseone film. In this embodiment, the superconductor film undergoes stressengineering as discussed below.

FIG. 3 shows the second of the either stress-engineered orsuperconductor films as 16. The superconductor film comprises a materialthat becomes superconducting at low temperatures. Example materialsinclude niobium, niobium titanium (NbTi), niobium nitride (NbN),tantalum, titanium nitride (TiN), magnesium dibromide (MgB₂), or yttriumbarium copper oxide (YBCO).

The stress-engineered film may result from many different processes. Asused here, a “stress-engineered” film means a film that has a stressgradient. In the embodiments here, the stress gradient in the filmresults in a film having higher compressive stress near the substratethan near the surface of the film. The film stress at the substrate maybe compressive, neutral, or tensile, as long as it is more compressivethan some overlaying film layers above. This discussion may refer to thestress-engineered film as an elastic material.

In one embodiment, compressively-stressed films can be attained bydepositing the film via physical vapor deposition (sputtering) in anambient with relatively low pressure. The low ambient pressure leads tofewer collisions with atoms in the ambient gas and to consequentlyhigher impact energies at the substrate, thereby leading to a morecompact, compressively stressed film. The electrical power applied tothe sputter target can be increased to further increase the impactenergy and promote compressive stress. Tensile stress, on the otherhand, can be attained by increasing the ambient pressure and decreasingthe electrical power applied to the target to lower the impact energy.

As mentioned above, the stress-engineered film and the superconductingfilm may be the same film. However, if they are different films, thestress-engineered film may comprise a molybdenum chromium (MoCr) film,resulting from using a MoCr target during sputtering. The two films maybe deposited using the same machine in one sequential sputtering processor may be deposited in separate processes in different machines. Forsome films, the deposition could be done using laser pulsed deposition,as is common for YBCO (yttrium barium copper oxide) superconductingfilms.

In one embodiment, a protective layer such as 18 shown in FIG. 4, mayalso reside in the stack of films. In summary, the stack of films 20 maycomprise one film of an elastic material that is both stress-engineeredand a superconductor, may include a protective film, such as gold, or atleast a partial gold film that does not cover the entire film upon whichit resides. The stack of films may also comprise two separate films, asuperconductor film and a stress-engineered, elastic material in anyorder. The elastic material may reside directly on the release film,with the superconductor film directly on the stress-engineered film orthe opposite. A gold film may be placed below, above, or on both sidesof the stress-engineered film. In many applications, a noble metal filmsuch as gold is desirable to protect the sandwiched films fromenvironmental gasses or chemicals. The discussion will refer to thiscoating as a protective coating, whether it actually protects theunderlying structure or not. The protective coating may comprise gold,nickel, copper, or combinations thereof. The discussion may refer tothese films as being “arranged on” the other films, or adjacent to theother films. No order is implied by these terms and are they are meantto encompass the films being in either order.

After manufacture of the stack of films, the process selectively removesthe release layer 12, leaving only anchor portions such as 13 shown inFIG. 5, to release the stack of films 20 from the substrate, however thestack is comprised. In FIG. 5, one can see that upon release, the stressgradient causes the stress-engineered film to curl up from the substratedue to the built-in stress gradient. The discussion here refers to thisas the spring curling out of the plane of the substrate.

Typically, the resulting spring may be as small as 4 micrometers (μm)wide on a 6 μm pitch to 200 μm wide. The springs may have lift heightsranging from just a few microns off the substrate to nearly 1 millimeterhigh. The film thickness could range from 5 nanometers to severalmicrons, depending upon the current density needed for the application.In addition, the superconducting film, if separate from thestress-engineered film, should be thin enough so it does not produceexcessive mechanical load that prevents the structure from properlycurling away from the substrate, while providing the needed thicknessfor the designed current density.

In another embodiment, the stack of films 20 could have thesuperconductor film deposited inside the middle of the elastic material.This may be accomplished after the first pressure part of the processbut before the second pressure part of the process. The resultingstructure may appear something similar to the structure shown in FIG. 6.In FIGS. 6, the superconductor film, in this embodiment film 16, lieswithin the stress-engineered film 14. The structure of FIG. 6 includes agold layer 18, which may or may not be included. In another embodiment,the superconductor film may envelop the elastic material, in which casethe inner film would be the elastic member and the superconductor filmwould be the outer film. When the springs act as interconnects forquantum computing chips, the superconducting film may or may not makedirect contact with the package to which they are connecting. If thesuperconducting layer lies on the top of the stack of film, it will makedirect contact. If a protective film lies on top of the stack of films,the superconducting film may connect through the gold layer.Alternatively, the contact area may be a wide flattened region, insteadof a sharp tip.

FIG. 7 shows a released thin film structure that utilizes broad backsidemetal areas such as 22 to form contacts. These broad backside contactareas such as 22 can be attained for example, by letting the spring tipcurl around their radius of curvature so the backside of the spring isat the highest point that contacts a pad. The spring shape can bemanipulated by adding straight segments with large radius of curvaturesor segments with reversed curvature that bends the spring in theopposite direction. These segments can be formed by adding mechanicalload layers or by depositing films of the opposite stress gradient onthose areas.

FIG. 8 shows an embodiment of a stack of films 20 that curl out of theplane of the substrate past 90 degrees to provide the backside metalareas such as 22.

The result has a substrate with a spring. The spring structure has ananchor portion disposed on the substrate. The spring structure has anelastic material having an intrinsic stress profile that biases a regionof the elastic material to curl away from the substrate, and asuperconductor film in electrical contact with a portion of the elasticmaterial. The region may be the tip of the spring or a region of theelastic material between the anchor portion and the tip, such as thatshown in FIGS. 7 and 8.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A method of manufacturing superconductorstructures, comprising: depositing a release film on a substrate;forming a stack of films comprising an elastic material and asuperconductor film; and releasing a portion of the elastic material byselective removal of the release film so the portion lifts out of thesubstrate plane to form elastic springs.
 2. The method as claimed inclaim 1, wherein forming the stack of films comprises depositing theelastic material above the release film and the superconductor filmabove the elastic material.
 3. The method as claimed in claim 1, whereinforming the stack of films comprises depositing the superconductor filmabove the release film and the elastic material above the superconductorfilm.
 4. The method as claimed in claim 1, wherein forming the stack offilms comprises: depositing a first portion of the elastic materialabove the release film; depositing a superconductor film above the firstportion of the elastic material; and depositing a second portion of theelastic material above the superconductor film, wherein the firstportion of the elastic material has intrinsic compressive stress and thesecond portion of the elastic material has intrinsic tensile stress. 5.The method as claimed in claim 1, wherein forming the stack of filmsincludes adding at least a partial layer of gold.
 6. The method asclaimed in claim 1, wherein the forming the stack of films comprisesdepositing the superconductor film within the elastic material.
 7. Themethod as claimed in claim 1, wherein forming the stack of filmsincludes forming the elastic material by sputtering one or morematerials under varying conditions to form a film having a region ofhigher compressive stress nearer to the substrate than at a surface ofthe film and a region of higher tensile stress at the surface thannearer to the substrate.
 8. The method as claimed in claim 7, whereinthe varying conditions comprises using lower pressure to form the regionof higher compressive stress for a first time and higher pressure toform the region of higher tensile strength for a second time.
 9. Themethod as claimed in claim 7, wherein the superconductor film isdeposited between the first time and the second time.
 10. The method asclaimed in claim 1, wherein forming the stack of films comprises usingone process of deposition for both the elastic material and thesuperconductor film.
 11. The method as claimed in claim 1, whereinforming the stack of films comprises depositing the elastic material andthe superconductor film in separate processes.
 12. The method as claimedin claim 1, wherein forming the stack of films comprises forming thesuperconductor film by laser pulsed deposition.
 13. The method asclaimed in claim 1, wherein forming the stack of films comprises forminga single film that is stress-engineered and a superconductor.
 14. Amethod of manufacturing superconductor structures, comprising:depositing a release film on a substrate; forming a stack of filmscomprising at least an elastic material; releasing a portion of theelastic material so that portion lifts out of a plane of the substrateto form elastic springs; and coating the elastic springs with asuperconductor film.
 15. The method of claim 14, wherein coating theelastic springs comprises one of sputter deposition, electroplating,electro-less plating, or atomic layer deposition.
 16. The method ofclaim 14, wherein coating the elastic springs with a superconductingfilm comprises coating the elastic springs with one of the groupconsisting of: niobium, niobium titanium (NbTi), niobium nitride (NbN),tantalum, titanium nitride (TiN), magnesium dibromide (MgB2), or yttriumbarium copper oxide (YBCO).
 17. The method of claim 14, furthercomprising coating the superconducting film with a protective coating.18. The method of claim 17, wherein coating the superconducting filmwith a protective coating comprises coating the superconducting filewith one of the group consisting of: gold, nickel, copper, orcombinations thereof.